1. Field of the Invention
The present invention relates to liquid crystal displays, and more particularly to methods for forming a stable alignment layer created by ion beam irradiation of a carbon film.
2. Description of the Related Art
Liquid crystal (LC) material employed in liquid crystal displays typically rely on alignment layers to establish a stable pretilt angle and other parameters such as anchoring energy for the liquid crystal material. Typically, the alignment of the liquid crystals for flat panel liquid crystal displays (LCD) is accomplished by placing a thin film of LC material on a mechanically rubbed polyimide film coated on a suitable substrate. Limitations imposed by the mechanical rubbing method (e.g., creating multiple domains for improving the viewing angle) in conjunction with the difficulty of optimizing polymer materials (e.g., polymers that avoid image sticking) make it highly desirable to use alternative materials and a non-contact LC alignment method.
There are a number of different methods/materials which have been shown to create LC alignment besides rubbing, for example, a stretched polymer, a Langmuir Blodgett film, a grating structure produced by microlithography, oblique angle deposition of silicon oxide, and polarized ultraviolet (UV) irradiation of a polymer film.
Non-contact methods to replace rubbing are described in commonly assigned U.S. Pat. No. 5,770,826, which describes a particularly attractive and versatile LC alignment process based on ion beam irradiation of a polyimide surface. The method places the LCs on a polyimide surface which has been bombarded with low energy (about 100 eV) Ar+ ions. This process has many characteristics which make it suitable for the manufacture of LC displays. This method has been extended to include diamond-like carbon (DLC), amorphous hydrogenated silicon, SiC, SiO2, glass, Si3N4, Al2O3, CeO2, SnO2, and ZnTiO2 films as described in commonly assigned U.S. Pat. No. 6,020,946. Another method for creating an LC alignment layer in a single deposition process has been described in commonly assigned U.S. Pat. No. 6,061,114.
Ion-beam treatment on DLC films (IB/DLC) for the alignment of liquid crystals has many advantages over conventional rubbed polyimide alignment, such as, non-contact processing, alignment uniformity, etc. Usually, DLC films of about 50 angstroms thick are deposited by plasma enhanced chemical vapor deposition (PECVD), and followed by Ar ion beam irradiation. It is believed that the Ar ion beam destroys the amorphous-carbon rings which have a large collision cross section to the ion beam. The amorphous-carbon rings which have a small or zero collision cross section to the ion beam are preserved. The average direction of the remaining carbon rings align the liquid crystal and generate a pretilt angle. The pretilt angle of IB/DLC alignment is not stable. The pretilt angle tends to decrease when the IB/DLC substrates are in contact with moisture or other components in air. The pretilt angle decreases as a function of storage time in vacuum-sealed LC cells with IB/DLC alignment. In addition, the pretilt angle is not stable under ultra-violet (UV) or violet irradiation. After ion-beam treatment, the surface of the DLC films are very active due to the ion-beam induced free radicals on the DLC surface. These free radicals tend to react with many chemical species in contact with them. This reaction may change the surface chemistry of the DLC film or change the orientation the carbon rings. As a result, the pretilt angle will degrade.
Therefore, a need exists for a non-contact alignment layer with a stable pretilt angle for use with liquid crystal displays.
Methods for inducing chemical changes to an alignment layer surface are described herein. Surface modification to an alignment layer surface, such as for example, a diamond like carbon (DLC) surface is carried out to enhance time stability of liquid crystal pretilt angle and further to fine-tune the pretilt angle and other properties related to the alignment of liquid crystal (LC) through interaction of the liquid crystal with the alignment layer surface.
Surface modification in accordance with the present invention may be grouped into several general categories for illustrative purposes. One category includes contemporaneous treatment of the surface alignment layer during the ion beam (IB) treatment. Reactive gas components may be added to the Argon gas normally used for the ion beam process, or the Argon gas may be replaced all together with reactive gas components. Such treatments could include, for example, hydrogen gas, fluorine gas, nitrogen gas, oxygen gas or larger molecules, such as, tetrafluoromethane. Free radicals and other types of reactive sites generated on the alignment layer surface (often called dangling bonds) would be immediately available for reaction with reactive species in the same environment. Dangling bonds on the surface would be able to be satisfied faster and in a controllable fashion using this method. Further, the reactive gases themselves may be activated by bond cleavage in the chamber creating in-situ atomic species such as, for example, atomic hydrogen or atomic nitrogen.
Another category includes post IB treatment of the alignment layer. A wet chemical, plasma, atom beam, or gas phase treatment may be employed to react with dangling bonds after IB treatment. Dangling bonds or other reactive sites created during the IB treatment could be reacted with these species resulting in a reproducible surface chemistry while maintaining the surface anisotropy desired for subsequent liquid crystal alignment.
A method for preparing an alignment surface of an ion beam processed alignment layer is described. Either concurrently with or subsequent to ion beam treatment of the surface, other chemical species are introduced and permitted to react to passivate dangling bonds. This stabilizes liquid crystal in a display to resist long term change and additionally permits fine-tuning of surface character and fine-tuning of the interaction between the liquid crystal and the prepared surface. Treatments include addition of gasses simultaneously with ion beam processing, or subsequently with gasses, plasmas, atom gun sources, or liquids.
A method for preparing an alignment layer surface, in accordance with the invention, includes providing a surface on the alignment layer, bombarding the surface with ions; and introducing reactive gas to the ion beam to saturate dangling bonds on the surface.
In other methods, the alignment layer may include diamond like carbon. The step of introducing reactive gas components may include the step of introducing one or more of nitrogen, hydrogen, oxygen, fluorine, silane, tetrafluoromethane as the reactive gas. The step of bombarding the surface with ions may include the step of bombarding the surface with Argon ions and reactive gas ions.
Another method for preparing an alignment layer surface, in accordance with the invention, includes the steps of providing a surface on the alignment layer, bombarding the surface with ions and quenching the surface with a reactive component to saturate dangling bonds on the surface.
In other methods, the alignment layer may include diamond like carbon. The step of quenching the surface with a reactive component may include the step of quenching the surface with a reactive gas to saturate dangling bonds on the surface. The reactive gas may include at least one of hydrogen, nitrogen, carbon dioxide, oxygen and water vapor. The step of quenching the surface with a reactive component may include the step of quenching the surface with a reactive liquid to saturate dangling bonds on the surface. The reactive liquid may include at least one of alcohol, water, hydrogen peroxide, carbon dioxide-saturated water, and liquid crystal. The step of quenching the surface may include the step of introducing atomic species to the surface by employing an atom gun. The atomic species may include one of hydrogen, nitrogen and oxygen.
Another method for preparing an alignment layer surface for liquid crystal displays, in accordance with the present invention, includes the steps of providing a diamond like carbon surface, bombarding the surface with ions from an ion beam, and saturating dangling bonds on the surface caused by the bombarding step.
In other methods, the step of bombarding may include the step of introducing a reactive gas to the ion beam. The reactive gas may include at least one of nitrogen, hydrogen, oxygen, fluorine silane and tetrafluoromethane. The step of bombarding the surface with ions may include the step of bombarding the surface with Argon ions and reactive gas ions. The step of saturating dangling bonds may include the step of quenching the surface with a reactive gas to saturate dangling bonds on the surface. The reactive gas may include at least one of hydrogen, nitrogen, carbon dioxide, oxygen and water vapor. The step of saturating dangling bonds may include the step of quenching the surface with a reactive liquid to saturate dangling bonds on the surface. The reactive liquid may include at least one of alcohol, water, hydrogen peroxide, carbon dioxide-saturated water, and liquid crystal. The step of saturating dangling bonds may include the step of introducing atomic species to the surface by employing an atom gun. The atomic species may include one of hydrogen, nitrogen and oxygen.
These and other objects, features and advantages of the present invention will become apparent from the following detailed description of illustrative embodiments thereof, which is to be read in connection with the accompanying drawings.